The U.K. was the likely source of a series of attacks last week that took down popular Web sites in the U.S. and South Korea, according to an analysis performed by a Vietnamese computer security analyst.

The results contradict assertions made by some in the U.S. and South Korean governments that North Korea was behind the attack. Security analysts had been skeptical of the claims, which were reportedly made in off-the-record briefings and for which proof was never delivered.